smugness versus arrogance

What is the meaning of "oily smugness" if compared to "brash arrogance"?

Donald Trump’s voice in foreign affairs is one that slips between brash arrogance and oily smugness. He touts supremacy from behind thinly concealed contempt. In withdrawing America from the Iran nuclearagreement, officially known as the Joint Comprehensive Plan of Action (JCPOA), Mr Trump risks pushing Iran out into the cold, triggering a nuclear arms race in the Middle East and handing power to the hardliners in Tehran. In place of the UN-approved deal is little more than Mr Trump’s bombastic promises of greater American independence and fewer unnecessary constraints.